In recent years, with the spread of fuel injection devices called injectors, the control of timing of injecting fuel and amount of fuel that is injected or air-fuel ratio has been getting easier, and as a result, it becomes possible to promote the realization of higher outputs, lower fuel consumption and cleaner exhaust emissions. Of these controlled items, in particular, as to the fuel injection timing, it is general practice to detect, strictly speaking, the condition of an inlet valve or, generally speaking, the phase condition of a camshaft and then to inject fuel to the result of the detection. However, a so-called camshaft sensor for detecting the phase condition of the camshaft is expensive and results in enlargement of a cylinder head when attempted to be fitted on, in particular, motorcycles, and as a result of these problems, the camshaft sensor cannot be adopted on motorcycles. Due to this, JP-A-10-227252, for example, proposes an engine control system for detecting the phase condition of a crankshaft and the pressure of induction air and then detecting the stroke condition in a cylinder from the results of the detections. Consequently, since the stroke condition can be detected without detecting the phase of the camshaft by using the conventional technique, it becomes possible to control the timing of injecting fuel to the stroke condition so detected.
Incidentally, in order to control the injection amount of fuel injected from the aforesaid fuel injection device, a target air-fuel ratio is set in accordance with, for example, engine rotational speed and throttle opening, an actual amount of induction air is detected, and the detected induction air amount is multiplied by the reciprocal ratio of the target air-fuel ratio, whereby a target fuel injection amount can be calculated.
While, in detecting the induction air amount, hot-wire airflow sensors and Karman vortex sensors are generally used as sensors for measuring mass flow and volume flow rate, respectively, a volume unit (a surge tank) for suppressing pressure pulsation is needed to eliminate a main factor for errors resulting from a reverse airflow, or the sensors need to be mounted on positions which are free from the entry of reverse airflow. However, in many engines for motorcycles, an intake system to each cylinder is a so-called independent intake system, or an engine itself is a single-cylinder engine, and in many cases the required conditions cannot be satisfied, and the induction air amount cannot be detected accurately even with these flow rate sensors.
In addition, an induction air amount is detected toward the end of an induction stroke or the beginning of a compression stroke, and since fuel has already been injected then, the control of air-fuel ratio using this induction air amount can only be implemented on the following cycle. This causes a rider to feel a feeling of physical disorder of not obtaining a sufficient acceleration because a torque and output that meet an acceleration which the rider has attempted to obtain by opening the throttle cannot be obtained until the following cycle even if the rider attempts to due to the control of air-fuel ratio being implemented based on the previous target air-fuel ratio. With a view to solving the problem, the intention of the rider to accelerate may be detected using a throttle valve sensor or a throttle position sensor for detecting the condition of the throttle, but, in the case of motorcycles, in particular, these sensors cannot be adopted since they are large in size and expensive, and therefore, the problem has not yet been solved currently.
The invention was developed to solve the problems and provides an engine control system which can obtain a sufficient acceleration by controlling the air-fuel ratio by detecting the intention of the rider to accelerate without using a throttle valve sensor or a throttle position sensor.